well "spirit magic" would logically have come from a living creature, so wouldnt it make sense that this sort of magic would flow most freely through what it is used to and developed in?
Sure oragnic matter is all jumbled up, but it is also arranged in such a way to allow for information and matter to pass through it freely. Viens were organized out of this jumbled matter to allow blood to flow.
Organic matter is more like a machine than a random assortment of crap. From afar it may looked jumbled, but its really a complex network of inter-twined mechanisms transfering information and matter in an efficient manner.
Also, I picture magic as a form of energy. And energy has no problem at all flowing through organic matter. Hell I've been electricuted too many times to not know that.